The advantage of RFID transponders compared to the bar-codes predominantly used in particular in the goods marking sector is that they permit a direct exchange of information, meaning that no visual contact is needed between the interrogating device and the transponder for the information transfer. Unlike bar-codes, furthermore, it is an easy matter with RF/ID transponders to change their information content directly as and when required.
RF/ID transponders find application in the most diverse fields. For example, they lend themselves to employment particularly in the fields of production, further processing, handling and protection of merchandise as well as in the personal security sector and the identification marking of animals.
The disadvantage of the known RF/ID transponders compared to bar-codes is the immense difference in price between the two identifiers. This is also the reason why the use of RF/ID transponders in the sales sector has been restricted so far to fringe areas. In particular there has been no interest to date in using RF/ID transponders to present price information or other data on mass products in department stores and warehouses because of the relatively high cost involved.
A method of bonding an integrated circuit to a resonant circuit mounted on a flexible substrate and a corresponding transponder are described in European Patent EP 0 821 406 A1. The resonant circuit is formed of two electrically conductive patterns arranged on either side of the flexible substrate. The resonant circuit has a predetermined inductance and a predetermined capacitance.
To ensure a reliable bond between the integrated circuit and the resonant circuit, the bonding area on the flexible substrate is first cleaned. Then the integrated circuit is connected to the resonant circuit by a technique referred to as wire bonding which is known from semiconductor technology. To prevent the bond from being destroyed by external impact, a protective coating is applied to the integrated circuit and the bonding area.
One disadvantage of this known method and known transponder resides in the relatively high production cost. A factor contributing to the high cost is also the absolute need to apply subsequently a protective coating to the integrated circuit and in particular to the bonding area between the integrated circuit and the resonant circuit, because otherwise the risk exists that the bond could be destroyed during use.
For many applications it is absolutely necessary for the transponders to be readily integrable into labels. Because the transponder according to the known solution is substantially thicker in the area of the chip than in the remaining areas, this presents major printing problems when printing labels with a label printer. A further disadvantage of the known solution resides in that the conducting tracks forming the resonant circuit are produced by etching. Etching is a relatively expensive process which, in addition, poses a severe environmental pollution problem.
From European Paten EP 0 595 549 B1 a remote-readable identification tag has become known in which electrical contact between the chip and the antenna is established by means of an electrically conducting adhesive (flip-chip bonding). In this type of bonding isotropic and anisotropic electrically conductive adhesive substances find application. To this end, the electrically conductive adhesive substance is applied to the antenna in the bonding area. The bonding areas of the chip are pressed into the electrically conductive adhesive; subsequently the adhesive is allowed to cure by the application of heat.
The problem involved in this method is that it is necessary to effect a metered application of the electrically conductive adhesive to a locally defined site on the antenna structure. Application is typically effected by screen printing which is a disadvantage because remnants of the expensive electrically conductive adhesive invariably remain in the application screens as residues.
Furthermore, curing of the adhesives requires a certain amount of energy. Considering, however, that for the heat treatment only limited maximum temperatures may be used, this has an adverse effect on the cycle time required for curing the adhesive substances. The result is again the undesirably high production cost.
One variant of flip-chip bonding utilizes adhesives in which electrically conducting particles are distributed at random in such low concentration that the adhesive is practically non conducting (anisotropic adhesives). As electrically conductive particles precious metals as, for example, palladium, silver or gold are used. Gold or nickel plated plastic pellets are also known, however. On account of the relatively large surface available and the elastic properties of the plastic material used, pellets have favorable bonding properties because of the availability of a correspondingly large bonding surface and also because the elastic restoring forces cause an intimate contact to be established between the conducting track and the surface of the contact points. In cases where also the chip contact points are made of a precious metal as, for example, gold, and the chip contact points are subsequently pressed onto the antenna through the adhesive substance, it will be obvious that also such electrically conducting particles distributed in the adhesive are present between the respective sites on the chip and the antenna needing to be bonded, hence ensuring the intended electrical connection. An undeniable disadvantage of flip-chip bonding is the relatively high expense for the adhesive substances.
A problem occurring in particular when using aluminum as a material for the antenna coil is the oxide layer developing on the surface of the conducting tracks as soon as they come into contact with the oxygen in the air. Aluminum oxide being a very hard substance and known as an insulator, it is of course highly undesirable in connection with bonding.